Statement of the Technical Field
The technical field of this disclosure comprises electrical interconnects, and more particularly concerns systems and methods for coaxial to planar microwave radio frequency interconnections in radio frequency systems.
Description of the Related Art
Radio Frequency (RF) circuits often require interconnects between the inner or center conductor of a coaxial transmission cable and planar entities which are disposed on a Printed Wiring Board (PWB). For example, such interconnects are commonly utilized where a coaxial cable enters an electronics module or chassis containing a PWB with conductive traces
The electrical interconnect between the inner conductor and the planar entity can create numerous design challenges. In part this is due to the competing requirements for the electrical and mechanical performance of the interconnect. For example, the interconnect will ideally facilitate an electrical connection having a consistent electrical frequency response. But from a mechanical perspective, the interconnect must be easy to fabricate and enable a low-cost highly repeatable assembly process. The interconnect and implementation should also be capable of accommodating differential thermo-elastic strains resulting from the inherent use of disparate coefficient-of-thermal-expansion (CTE) materials in the package design. Such strains can have adverse effects on electrical performance, mechanical durability, and interconnect life.
Conventional solutions for connecting planar printed wiring board traces to an inner conductor of the coaxial transmission line have involved various approaches. For low-rate custom pieces, a formed-in-place wire appliance can be fabricated by hand to connect a center conductor of the coaxial cable to the planar transmission line trace. But this method is labor intensive and it can be difficult to consistently produce connections which exhibit the same frequency response. Another solution involves machine formed wire or ribbon bonds between the planar entity and the center pin of the coaxial cable. However, these approaches can involve significant process variability and usually require special finishes and/or special processing equipment/tooling. Processing equipment and special tooling can be limited in their ability to physically access certain interconnect sites, which can restrict potential RF circuit design options. The wire or ribbon bonding methods can also involve onerous process development and durability validation testing.
A third alternative to the problem involves the use of a formed sheet metal appliance to facilitate the electrical connection between the coaxial cable center conductor and the planar entity. But conventional sheet metal appliances used for this purpose can have inconsistent supplier controlled geometries and conventionally involve complex flat-to-circular transitions. These conventional formed-metal appliances can also present issues with respect to durability and process requirements. Inconsistency of form and lack of inherent strain relief with traditional appliances often results in durability issues. Due to the complexity of forms there is limited availability of suppliers to tailor to specific applications. Plating and finish configurations typically available in the marketplace obligate certain design and processing constraints to ensure product reliability.